Component solderability is an industry wide problem which, if not detected, can result in excessive board joint rework and system failures. In the Defense Industry, the majority of military contracts require that samples from component lots be inspected for solderability per MIL-STD-202, Method 208. This inspection task is highly labor intensive and susceptible to human error.
Typically, samples of the components to be tested are selected and tested under military specified conditions to gauge the solder wetting ability on the component leads. MIL-STD-202, Method 208 specifies the operational requirements for all the equipment to be used in the test and the evaluation criteria to determine if the solder dipped components pass or fail. The critical area of the evaluation process is the visual examination of the solder coverage on the leads after the solder dip operation. The requirements specify that a specially trained and certified inspector perform a "visual" measurement of the solder coating at 10 power magnification with a "shadowless" light source. The 1-inch portion of the dipped lead nearest the component, or the whole lead if it is less than 1 inch long, is examined. If the new solder coating doesn't cover at least 95% of the lead then the specimen has failed the test. Visual determination of the solder coverage makes it very hard to get accurate and repeatable results. Even though a limited number of components are selected from each lot, the inspection operation through a microscope is very tedious. The only "tool" that has been available to help in this measurement is a grid reticle that fits in the eyepiece of a microscope. This requires the inspector to count the squares that the defects occupy and gauge against the 95% coverage criteria. Unfortunately, the grid is not very effective at the required 10x magnification.
The examination of each component requires inspectors to visually measure the total solder coverage. Components failing to meet a minimum of 95% solder coverage or components with large defect regions are rejected. When the length or diameter of the leads changes, it is even more difficult for the inspector to gauge the percent coverage. Another problem in the inspection process is the inspection lighting. A number of lighting techniques commonly used, such as ring lights and multi-point light sources, make it very difficult to identify surface defects on the lead.